Optical fiber used in communication systems typically includes a glass core surrounded by a cladding that is also formed from glass. The glass cladding and the glass core have different optical properties. Typically, one or more protective coating layers surround the core and cladding. Such fibers can be made by drawing a thin strand from a heated, partially molten glass preform having the proper composition to cause the cladding surrounding the core to have the proper composition. As a strand of soft, molten glass is pulled from the preform, both the core glass and the cladding glass stretch. During this process, the core remains in the middle of the fiber and the cladding remains on the outside, thus forming the composite core and cladding structure of the finished fiber. As the fiber is pulled away from the preform, it cools and solidifies, and the coating is applied. These processes normally are performed at high speeds so that the fiber is drawn at high rates.
In optical fiber communications systems, light injected into one end of the fiber is pulsed, or progressively varied, in accordance with information to be transmitted over the system. The speed at which light propagates along a fiber depends upon many factors, including the optical properties of the materials that make up the core and cladding and the diameter of the core. Light passing along the fiber typically includes portions of light having different polarizations, i.e., different orientations of the electromagnetic waves constituting the light. If the fiber core is not perfectly cylindrical, but instead has long and short diameters, light of one polarization will have its electrical waves aligned with a long diameter of the core whereas light of another polarization will have its electrical waves aligned with the short diameter of the core. In this case, the effective diameter of the fiber core will be different for light of one polarization than for light of another polarization. Portions of light having different polarizations will travel at different velocities. In addition to this geometrical effect, differential thermal expansion between the core and cladding gives rise to stress induced birefringence, which also increases the difference between propagation velocities of light of different polarizations. Stated another way, the fiber has a “slow” axis in one direction perpendicular to its length, and a “fast” axis in the other direction perpendicular to its length.
Light having a direction of polarization aligned with the fast axis travels more rapidly than light having a direction of polarization aligned with the slow axis. As a result, the two polarization modes propagate with different propagation constants. The difference between the propagation constants is termed birefringence, and the magnitude of the birefringence is given by the difference between the propagation constants of the two orthogonal modes. Birefringence causes the polarization state of light propagating in the fiber to evolve periodically along the length of the fiber. The distance required for the polarization to return to its original state is typically referred to as the fiber beat length, which is inversely proportional to the fiber birefringence. Accordingly, fibers with more birefringence have shorter beat lengths, and vice versa.
In addition to causing periodic changes in the polarization state of light traveling in a fiber, the presence of birefringence means that the two polarization modes travel at different group velocities, with the difference increasing as the birefringence increases. The differential time delay between the two polarization modes is called polarization mode dispersion, or PMD. Imperfections in the fiber other than core shape can also contribute to PMD. PMD distorts the light pulses or waves transmitted along the fiber, thus reducing the signal quality and limiting the rate at which information can be passed along the fiber. PMD is very harmful to signal quality for high bit rate systems and analog communication systems.
One way to reduce the effects of PMD is to continually re-orient the fast and slow axes of the fiber. This can be accomplished by spinning the fiber as it is drawn so that the slow axis and the fast axis of the fiber are repeatedly interchanged along the length of the fiber. Thus, at one point along the length of the fiber the slow axis points in a first direction perpendicular to the length of the fiber and the fast axis points in a second direction perpendicular to the length of the fiber and to the first direction. At another point along the length of the fiber, the fast axis points in the first direction and the slow axis points in the second direction. The specifics regarding reduction in PMD achieved by spinning the fiber depend on the detailed variation of the orientation of the fast and slow axes along the length of the fiber, and so it is often desired to have a particular pattern of spin imposed along the length of a fiber.
Therefore, the manner in which fiber is spun during the drawing process should be monitored because, for various reasons, at times the device that spins the fiber malfunctions or performs improperly. The spin device is located on the draw tower and some spin devices are variable so that if a determination is made that the amount of spin is improper, the spin device can be adjusted to correct the spin. A known technique for measuring the spin in the fiber generally utilizes polarization characteristics of a length of fiber being tested to measure spin. The known method couples polarized light into a short length of fiber (e.g., two meters) and aligns a polarizer on one end of the piece of fiber with a polarization adjuster on the other end. A particular length of the fiber is cut off (e.g., one centimeter), the polarizer and the polarization analyzer are realigned and the angle of the polarization analyzer is recorded. The angular position of the polarization analyzer is directly related to the spin fixed in the glass. The process is very slow and would appear to be impractical to automate.
Accordingly, a need exists for a method and apparatus for accurately measuring the spin in an optical fibers.